Joshua Keating is a staff writer at Slate focusing on international affairs.

The index defines slavery as “the possession and control of a person in such a way as to significantly deprive that person of his or her individual liberty, with the intent of exploiting that person through their use management profit transfer or disposal.” To a lesser extent, the index also measures factors like human trafficking and early child marriage.

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While no countries are entirely free of slavery—there are between 57,000 and 63,000 enslaved people living in the United States today according to the index—the problem is particularly acute in some places. Topping the index is the West African nation of Mauritania, where there are between 140,000 and 160,000 slaves out of a population of just 3.8 million:

Slavery in Mauritania primarily takes the form of chattel slavery, meaning that adults and children in slavery are the full property of their masters who exercise total ownership over them and their descendants. Slave status has been passed down through the generations from people originally captured during historical raids by the slave-owning groups. People in slavery may be bought and sold, rented out and given away as gifts. Slavery is prevalent in both rural and urban areas. It is reported that women are disproportionately affected by slavery; for example, they usually work within the domestic sphere, and a high level of control is exercised over their movements and social interactions.

The index’s estimate for Mauritania is actually on the low side. Other NGOs have estimated that slavery may effect up to 20 percent of the population. Mauritania was the last country in the world to officially abolish slavery in 1981.

Slavery is also a serious problem in Haiti, where children are often exploited through a system of child labor called “restavek,” in which “disadvantaged children from rural areas are sent to work as domestic helpers for wealthier families.” While lower on the prevalence scale, India and China have the highest total populations living in modern slavery. Nearly half the world’s slaves live in India.

While most of the countries topping the index are in Africa and Asia, Moldova—Europe’s poorest country and a major source country for migrants forced into “exploitation in the sex industry, construction, agriculture, and domestic work”—is ranked sixth.

The Walk Free Foundation was established last year by the Australian mining magnate Andrew Forrest, and the index has been endorsed by figures including Hillary Clinton, Bill Gates, Mo Ibrahimm, and Mohamed Yunus.

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